1. Field of the Invention
This invention relates to a system for controlling the locomotion of a legged walking robot and, more particularly, to a system for controlling the locomotion of a biped walking robot or the like which is able to walk stably over rough terrain.
2. Description of the Prior Art
Two methods have been developed for achieving walking of a legged walking robot. One is the method disclosed, for example, in Japanese Laid-open Patent Publication No. 62-97006 in which data for driving the robot's leg joints is calculated in advance using a large off-board computer and stored in the memory of a computer mounted on the robot for output by the computer during robot locomotion. The other is the method described in, for example, "Legged Robots on Rough Terrain: Experiments in Adjusting Step Length: IEEE 1988, by Jessica Hodgins" in which the robot is equipped with a high-performance on-board computer which calculates the optimum joint angles in real time during robot locomotion and controls the joint drive motors to obtain the calculated angles.
While the first method has the advantage of not requiring a sophisticated on-board computer, the fact that the locomotion data is calculated in advance makes it workable only if the conditions encountered during actual walking are completely identical to those envisioned in advance. The method is thus not capable of coping with unforeseen conditions arising in the course of actual walking and, in particular, cannot effectively deal with unexpected undulations and irregularities in the terrain being traversed. Having a high center of gravity and feet with small ground contact areas, a biped robot is physically unstable. For such a robot to walk stably, it is necessary to ensure that the resultant of the weight acting on the robot's center of gravity and the inertial force falls within the ground contact area of the supporting leg. While the weight does not change, the robot's locomotion causes the inertial force to vary constantly in a pattern which repeats in time and space at regular periods insofar as the conditions external to the robot remain constant. The conditions external of the robot are, however, not constant and there are moment-to-moment changes in the terrain in the form of varying inclination, undulations, bumps and the like, as well as changes in the terrain hardness and other physical factors. Since these variations cause changes in the reactive force the robot receives at the time of free leg footfall, they destabilize the robots locomotion. While of less importance than the external condition changes, changes in the internal conditions of the robot can also have a destabilizing effect. For example, friction at the joints varies with the state of shock absorption, and the flexing of mechanical components due to insufficient stiffness can also have an effect. It is impossible to cope with such changes using pre-calculated data.
In stabilizing locomotion in the second method, once the state of the robot has been ascertained, it is then necessary to conduct a series of real-time processing operations which include solving highly complex determinants for deciding the gait, calculating optimum location for the next foot placement and driving the free leg for actually placing the foot at the calculated location. In the case of the biped walking robot whose legs each have six degrees of freedom therefore, the large determinants that have to be solved for conducting the control make it necessary to use a very large, high-performance computer. Thus the second method requires the robot to be equipped with a high-speed, high-performance computer, its power consumption is high and the computer itself is large, heavy and expensive. These are disadvantages that are hard to tolerate in a walking robot.
The first object of this invention is therefore to provide a system for controlling the locomotion of a legged walking robot which overcomes the aforesaid drawbacks of the prior art by retaining the advantages that the first method realizes by controlling walking on the basis of data calculated offline in advance while, at the same time, enabling the flexible response to unforeseen changes in locomotion conditions that is required for achieving stable walking.
There is no difference in the likelihood of changes arising in the terrain conditions between the first method of controlling locomotion on the basis of data calculate offline in advance and the second method of deciding the optimum gait in real time. Moreover, even the second method is not necessarily able to cope quickly enough with changes in the locomotion conditions that arise suddenly, and in this sense, therefore, the problems of the first method are also problems of the second method.
The second object of the invention is therefore to provided a system for controlling the locomotion of a legged walking robot which is able to stabilize the attitude of the robot by quickly coping with sudden changes in terrain conditions, irrespective of which of the aforesaid two methods is employed.
The third object of the invention is to provide a system for controlling the locomotion of a legged walking robot which overcomes the aforesaid drawbacks by combining the advantages of the aforesaid two methods and, more specifically, to provide such a system wherein the gait is set offline in advance and the set gait can be readily modified for restoring walking stability when unexpected walking conditions are encountered during actual walking.
The offline setting of a target gait in advance may involve not only the determination of detailed settings such as the aforesaid joint angles but also the setting of such higher level data such as the foot placement position and the position of the center of gravity. Moreover, even in a method which determines the gait in real time during walking, it is not possible to avoid being forced to modify the gait upon encountering sudden, unexpected changes in the walking conditions.
The fourth object of the invention is therefore to provide a system for controlling the locomotion of a legged walking robot enabling the gait to be readily modified in response to unexpected changes in the walking conditions irrespective of whether or not locomotion data has been set offline in advance.
Moreover, recent years have seen extensive research into stabilization methods for robots (including legged walking robots) in which the robot's linkages are subjected to kinematic analysis, the robot's behavior is reduced to mathematical equations using various control theories falling in the realm of modern control system theory, and the equations are solved for obtaining control values. However, although the use of modern control system theory makes it possible to obtain very precise control values, the large number of determinants that have to be solved for implementing this type of control require the robot to be equipped with a high-performance computer. Moreover, since it is impossible to fully ascertain the rigidity etc. of the linkages, the results obtained through the application of the theory have been small in proportion to the amount of effort made.
The fifth object of the invention is therefore to provide a system for controlling the locomotion of a legged walking robot which is capable quickly responding to unexpected changes in the locomotion conditions for restoring the postural stability of the robot in a simple manner and which can be implemented using a relatively low-performance on-board computer.